This invention relates to a method for controlling the heating elements of a thermal print head for recording and erasing dots with a reversibly writable thermal recording material.
A reversibly writable thermal recording material is characterized in that its transparency and/or color can change reversibly from a transparent and/or colorless state to an opaque and/or colored state and vice versa in dependence on temperature.
The reversibly writable thermal recording material is supplied step-by-step to the thermal print head. The print head has a row of individually drivable resistance heating elements extending over the total printing width transversely to the transport direction of the thermal recording material. In each print step one can record a line of dots corresponding to the row of heating elements if the heating elements are heated to a temperature leading to the colored/opaque state of the thermal recording material.
Erasure of the colored/opaque dots can be effected by a second thermal print head whose heating elements are heated to a temperature at which the reversibly writable thermal recording material changes back to the colorless/transparent state. One can also use a single thermal print head which erases when the recording material is moved along it in one direction, and records, i.e. writes dots, upon subsequent movement of the recording material in the reverse direction (DE 41 30 539 A1).
German Patent Document No. DE 42 10 379 C2 discloses first applying an energy pulse train to drive the heating elements that are to record a dot and then applying another energy pulse train to the heating elements that are to perform dot-by-dot erasure, in each transport cycle.
In known reversible recording methods, however, the recording speed leaves something to be desired.
The object of the invention is to substantially increase the recording and erase speed in thermal printing of a reversibly writable recording material.
According to the invention, the heating elements are driven for writing with a single energy pulse leading to a temperature at which the reversibly writable thermal recording material assumes a first, high temperature leading to the colored/opaque state.
The heating elements which are to perform erasure are then subjected to an energy pulse train when the maximum temperature has been reached after the recording pulse. This permits the processing, i.e. recording and erasure of the individual dots of a printed line, to be reduced to 3 milliseconds or less and an accordingly high recording and erase speed to be reached.
According to the invention, one uses a reversibly writable thermal recording material that becomes colored and/or opaque at the first, high temperature and retains the colored/opaque state upon rapid cooling. However, upon slow cooling, the colored/opaque state of this thermal recording material is lost if constant heating to a second lower temperature takes place.
The first high temperature that makes the thermal recording material become colored or opaque, i.e. milky, may be 150xc2x0 C. or more for example. The second lower temperature to be held constant leading to erasure is preferably at least 20xc2x0 C. lower.
Therefore, the heating elements can be subjected to the energy pulse train for erasure in two versions according to the invention.
According to one variation, all heating elements are first driven with the recording energy pulse and, subsequent to the recording energy pulse, an energy pulse train is supplied that slows down the cooling of those heating elements which are to bring about erasure such that the recording material assumes its colorless/transparent state. In this version, all heating elements are thus in each cycle first heated to the temperature necessary for coloring the recording material and the heating elements that are to erase dot-by-dot are then subjected to the pulse train in order to cool more slowly than the other heating elements. One need not necessarily drive all heating elements of the thermal print head in this fashion, but only those which correspond to the desired printing width. The colorless/transparent state might also have a different color from the one appearing upon coloring of the thermal recording material.
According to the second version of the invention, the heating elements for recording are subjected to the recording energy pulse and the heating elements for erasure, directly subsequent to the recording energy pulse, to an energy pulse train which heats the heating elements to a second temperature to be held constant at which the thermal recording material assumes a transparent/colorless state, the second temperature being below the temperature producing the colored/opaque state.
In the second version, however, the second temperature must in general be held for a certain time of at least 1 millisecond for erasure. It is therefore in general somewhat slower than the first variant. That is, the pulse duration for the recording pulse is approximately 1 to 2 milliseconds. Whereas, the duration of the pulse train supplied during cooling in the first variant is approximately 1 to 2 milliseconds, the duration of the pulse train for erasure in the second variant is approximately 2 to 3 milliseconds in order to hold the temperature for at least approximately 1 millisecond at the second temperature at which the thermal recording material assumes the transparent/colorless state.
The reversibly writable thermal recording material that can be used according to the invention may be any known reversibly writable thermal recording material (compare DE 41 30 539 A1, DE 42 10 379 C2 and 42 00 474 C2). However, one preferably uses a recording dialkylamine residue at the 3 position and at its 9 position a phenyl residue is bound with a carboxyl acid group at the ortho position so that, as in fluorescein, a lactone ring forms with the 9 position in the leuco form, said ring being open in the colored state through re-formation of the carboxyl group. As a developer, one can use an acid amide of carboxylic acid with a para-aminophenol and/or a urea derivative substituted with a para-hydroxyphenyl residue on an amino group and with an alkyl residue on the other amino group.
The energy supply for erasure in the form of a pulse train obtains fine temperature control according to the invention. For this purpose, the pulse train has pulses with the same period of preferably less than 100 microseconds, in particular less as 50 microseconds. The pulse/pause ratio per period is preferably at most 1:1, a maximum on duty cycle of 50%, in particular approximately 1:2, an on duty cycle of 33%. That is, at a period of e.g. 30 microseconds the pulse duration is 10 microseconds and the pause 20 microseconds for example.
Preferably, the heating elements of the thermal print head are preheated before processing, i.e. recording and erasure, to a temperature that is preferably at least 30xc2x0 C. below the second, i.e. erase, temperature. If the erase temperature is 120xc2x0 C. for example, the preheating temperature can be approximately 60xc2x0 C. for example.
Such preheating in thermal printing is indicated for example by DE 30 33 746 A1. Preheating lowers the temperature difference until recording or erasure, i.e. reduces the heating capacity necessary for printing,
Such preheating in thermal printing is indicated for example by DE 30 33 746 A1. Preheating lowers the temperature difference until recording or erasure, i.e. reduces the heating capacity necessary for printing, thereby achieving a higher printing speed due to the faster heating of the resistance heating elements. Moreover, the erase quality is clearly improved.
While, according to DE 38 33 746 A1, the clock frequency during preheating should be no more than the quadruple of the pulse duration for recording and the pulse width during preheating should be constant, according to the invention the period of the single pulses of the pulse train for preheating is less than 100 microseconds, in particular less than 50 microseconds, i.e. less than one tenth, preferably less than one twentieth, of the pulse duration at a pulse duration for the recording pulse of 1 to 2 milliseconds.
In order to permit the desired preheating temperature to be adjusted as exactly as possible, the pulse/pause ratio per period, the on duty cycle, is furthermore preferably reduced with increasing temperature of the thermal print head. Thus, at a constant period of the single pulses, the pulse duration can be for example 10% or less of the period at the beginning of preheating, and for example 3% or less at the end of the preheating process or for holding the preheating temperature. That is, at a period of for example 30 microseconds per single pulse, the pulse duration can be for example 2 microseconds at the beginning of preheating and for example 0.5 microseconds at the end of preheating and for holding the preheating temperature.
The pulse duration during preheating can be controlled for example by the temperature of the thermal print head, which can be measured with a temperature sensor, for example a temperature-dependent resistor with a negative temperature coefficient.
Under these circumstances, the preheating temperature of the heating elements can be adjusted to for example xc2x12xc2x0 C. or even more exactly. The thermal print head is thus minimally stressed thermally and its life essentially increased. As experiments indicate, this even makes the life longer than without preheating since the thermal print head is subject to smaller temperature jumps during recording. The period of the single pulses of the pulse train during preheating preferably corresponds to the period of the single pulses of the pulse train for erasure, being for example 30 microseconds in both cases.